project: Make time format a first-class citizen

The current code deals with 12/24h clock formats almost like
a workaround of the issue. This commit adds proper support to
different clock formats.

src/gcal-enums.h

@@ -18,6 +18,18 @@
 
 #pragma once
 
+#include "gcal-enum-types.h"
+
+/**
+ * GcalTimeFormat:
+ * @GCAL_TIME_FORMAT_12H: time is displayed in 12h format
+ * @GCAL_TIME_FORMAT_24H: time is displayed in 24h format
+ */
+typedef enum
+{
+  GCAL_TIME_FORMAT_12H,
+  GCAL_TIME_FORMAT_24H,
+} GcalTimeFormat;
 
 /**
  * GcalWindowViewType: